The spread of FTTH (Fiber To The Home) is progressing globally due to an increasing need for high-speed access services. Most FTTH services are provided by an economically excellent PON (Passive Optical Network) system, in which a single storage station-side device (OSU: Optical Subscriber Unit) stores a plurality of subscriber-side devices (ONU: Optical Network Unit) by time division multiplexing (TDM).
In an upstream communication by a TDM-PON system, the system bandwidth is shared between the ONUs based on a dynamic bandwidth allocation calculation in the OSU, and each ONU intermittently transmits an optical signal only within a transmission permission time period notified by the OSU, thereby preventing collision between optical signals.
The current main systems are GE-PON (Gigabit Ethernet (registered trademark) PON) and G-PON (Gigabit-capable PON), which have gigabit-level transmission speeds. In addition to the progress of video distribution services, the emergence of applications that upload/download large-capacity files requires further increases in the capacity of PON systems.
However, in the TDM-PON system described above, since the system bandwidth is expanded by increasing the line rate, the reception characteristics are greatly deteriorated due to the effects of higher speed and wavelength dispersion, and further, the economy of the burst transceiver becomes a problem, thereby making it difficult to increase the capacity to more than 10 gigabytes.
Application of a wavelength division multiplexing (WDM) technique is being investigated for increasing the capacity to more than 10 gigabytes. FIG. 1 is an example of a WDM/TDM-PON system in which a WDM technique is combined with a TDM-PON system related to the present invention.
The WDM/TDM-PON described in the present specification is synonymous with a basic system TWDM (Time and Wavelength Division Multiplexing)-PON of the Recommendation G.989 series internationally standardized in the ITU-T (International Telecommunication Union—Telecommunication Standardization Sector). Furthermore, the technique described in the present specification can also be applied to a WDM-PON.
The WDM/TDM-PON system shown in FIG. 1 includes OSUs 10 #1 to #M and a plurality of ONUs 93. The OSUs 10 #1 to #M respectively communicate with the plurality of ONUs 93 using a wavelength set of λU_1,D_1 to λU_M,D_M. Here, λU_1,D_1 indicates a combination of an upstream wavelength λU_1 of an upstream signal and a downstream wavelength λD_1 of a downstream signal.
Each ONU 93 is fixedly assigned a downstream wavelength and an upstream wavelength according to the terminal of a wavelength routing unit 94-1 to which it is connected. Temporal signal overlap among all ONUs 93 is permitted for #1 to #M, that is to say, up to the number of OSUs 10. Consequently, by adding an OSU 10, the system bandwidth can be expanded without increasing the line rate per wavelength.
Among the terminals of the wavelength routing unit 94-1, each ONU 93 connected to the same terminal on the ONU 93 side and connected to an optical fiber transmission line 96 is logically connected to the same OSU 10, and share an upstream bandwidth and a downstream bandwidth.
For example, the ONUs 93 #2-1 to #2-K are logically connected to the OSU 10 #2. Here, the logical connection between each ONU 93 and the OSU 10 is constant, and it is not possible to distribute traffic load among different OSUs 10 #1 to #M according to the state of traffic load of each OSU 10.
On the other hand, as shown in FIG. 2, proposed is a wavelength tunable WDM/TDM-PON system whose optical transmitter and optical receiver mounted on the ONU 93 are equipped with a wavelength tuning function (for example, refer to Non-Patent Document 1).
The ONU 93 includes a wavelength tunable light transmission unit 31, a wavelength tunable light reception unit 32 having a light receiving unit 321 and a wavelength tunable filter 322, and a wavelength multiplexing and demultiplexing unit 33.
In the configuration of Non-Patent Document 1, it is possible to individually change the logical connection destination OSU 10 of each ONU 93 by switching the transmission/reception wavelength in the ONU 93. As a result of using this function, when there is an OSU 10 in a high-load state, the logical connection between the ONU 93 and the OSU 10 is changed so that the traffic load is dispersed to an OSU 10 in a low-load state, and it is possible to prevent a deterioration in the communication quality of the OSU 10 in the high-load state.
Furthermore, when a high-load state of an OSU 10 regularly occurs, in the WDM/TDM-PON system of FIG. 1, it is necessary to add system bandwidth in order to ensure a fixed communication quality. In the wavelength tunable WDM/TDM-PON system of FIG. 2, it is possible to ensure a fixed communication quality by effectively utilizing the bandwidth of the entire system by distributing the traffic load among the OSUs 10, and capital investments for expanding the system bandwidth can be minimized.